Virology Websites

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Virology Websites
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Viruses Defined by Unique Properties

• Obligate intracellular parasites that do not
  undergo binary fission
• distinguish from bacterial pathogens and other
  parasites
• Parasitic genomes related to plasmids that can
  exist in two forms
• distinguish from other life forms

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Virus Life Cycle
Extracellular (metabolically inert)
Intracellular (replicating)
genome uncoated
Synthesis Replication
Virus particle (virion)
1. Genome (DNA or RNA)
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Definitions
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Plant Virus Life Cycle Adaptations to the Cell
Wall at Entry and Exit
entry/penetration
Attachment/Penetration animal viruses bind to
specific surface receptors Entry fuse with or
engulfed by the plasma membrane
genome uncoated
Plant viruses mechanical introduction
initially. NO evidence attach to specific
receptor sites on cell wall
(RNA cytoplasm DNA nucleus)
replication
mRNA
Transmission animal viruses aerosols, break in
skin, blood, sexual contact
proteins
plant viruses insects, fungi, nematodes,
abrasion, seeds, pollen, vegetative propagation
progeny virion assembly
Release animal viruses lyse cells or bud
through plasma membrane
plant viruses channel through wall (MPs)
without lysis
release
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There is no Evidence that Plant Viruses Attach to
Specific Receptor Sites on Plant Cells
The stages of plant virus penetration and
uncoating of the viral genome are not understood
Plant viruses are initially introduced through
the cell wall by mechanical means when a plant is
first infected
Studies with some plant viruses (e.g. TMV, TBSV)
suggest that bound Ca2 can stabilize virion
capsid structure
It has been suggested that the low Ca2
concentrations inside plant cells may facilitate
uncoating of at least some plant viruses
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Lazarowitz (2001) in Fundamental Virology (Knipe
Howley, eds. )4th Ed, Lippincott
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Virus Intercellular Transport Cell-to-Cell
Spread of Infection
The goal is for progeny viruses to infect new
host cells to repeat the infectious cycle

• May be immediate neighbors or distant cells

• In animals and humans, the circulatory or
  nervous systemic
• are conduits for systemic infection

All phage and the majority of animal viruses go
through an extracellular form (virions) to infect
additional cells
Plant viruses move directly cell-to-cell without
an extracellular phase within a leaf (local
movement)
An extracellular form of the viral genome is
transported leaf to leaf through the phloem (long
distance or systemic movement)
For most, but not all, plant viruses the
extracellular form appears to be virus particles
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Plant Viruses use Movement Proteins to Move Cell
to Cell without an Extracellular Phage and
without Lysis
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Classification of Viruses
Lwoff, Horne, and Tournier (1962)
Grouped by shared properties rather than the
properties of the cells or organisms they infect
Genome is primary criterion for classification
Characteristics used for classification

• Morphological distinctions belie similarities
  in genome organization
• and replication strategy of different viruses

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Classification of Plant Viruses
van Regenmortel et al. (1999) ICTV Report
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Baltimore Classification Scheme
Based on genetic system of each virus
Describes relationship between viral genome and
its mRNA
mRNA positive () sense (also DNA of
equivalent polarity)
complement negative () sense
Flint et al. Principles of Virology (1999)
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Infectious Subviral Agents
Prions (pathogens of animals)
Self-replicating proteins
Satellites (pathogens of animal and plants)
Encapsidated nucleic acid that depends on an
unrelated helper virus for its replication
If it encodes its coat protein, it is a
satellite virus
Viroids (pathogens of plants)
Unencapsidated, highly structured small RNA
(240-390 nt)
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Viroids are Highly Structured Small RNAs
From Agrios (1997) Plant Pathology 4th Ed.
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virions
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Virus Life Cycle See a problem and figure out
how to surmount it
Enters cells and multiplies, usually at the
expense of the host cell
Will use host cell machinery
What are the molecular and cellular events that
allow virus multiplication?
What steps are virus-specific (potential targets
for resistance)?
How do viruses modify host cells to promote their
own multiplication?
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RNA VIRUSES

• ADVANTAGE
• RNA-dependent RNA polymerases do not require a
  primer
• replication can be initiated at the ends of a
  linear molecule
• PROBLEMS
• Cells do not constituitively express the enzymes
  required to directly replicate or transcribe RNA
• virus must provide its own RNA-dependent RNA
  polymerase (replicase, transcriptase)
• Eukaryotic RNA viruses replicate in the cytoplasm
• exceptions influenza, retroviruses
• Do not contain same controlling elements as the
  host to regulate gene expression
• regulate gene expression in unique ways
• distinguish mRNAs from replication templates
• Viral genes organized and translated like
  host genes
• eukaryotic RNA viruses must reconcile genome
  structure with requirement for monocistronic mRNAs

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Virus Life Cycle
Viral genomes are compact

• Strategies to expand their coding capacity and
  regulate their gene expression

• Transcriptional and translational

• Multiple proteins from one gene (translation unit)

Viruses play by the rules

• Obey host rules for mRNA translation

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Reading
Lazarowitz (2001) The Plant Viruses, pp 1-9.
Agrios (1997) Plant Pathology, Ch. 14 pp 479-508